1. Field of the Invention
The present invention is directed to a method for producing a solid state image detector, and a solid state image detector of the type formed by a pixel matrix that is coated with a passivation layer and that is made of a semiconductor material, particularly an amorphous silicon and a scintillator layer for purposes of converting radiation incident on the scintillator layer, so that it can be processed by the pixel matrix, the pixel matrix being composed of a number of sub-detectors that are arranged side by side and that are referred to as panels.
2. Description of the Prior Art
Solid state image detectors of the above type are known and are based on an active matrix (readout matrix) in the form of the pixel matrix made of a semiconductor material, amorphous silicon (a-Si), for example. In a radiographic converter, a scintillator layer (made of cesium iodide, for example) transforms the image information into a radiation that can be processed by the pixel matrix. The image information is stored as electrical charge in the active pixels of the pixel matrix and is subsequently read out in analog form and converted into digital form, with dedicated electronics. X-ray quanta are transformed into radiation in the scintillator layer, the transformation being into light in the case of cesium iodide which is coupled to the pixels of the amorphous silicon, the pixels being photo diodes. This optical process must be optimized in order to achieve a good image quality. Good coupling of the scintillator layer to the pixel matrix is important, since a poor coupling negatively influences the most important parameters that are relevant with respect to the image quality. These parameters are the resolution which, in the frequency domain is quantified by means of the MTF (modulation transfer function), and the frequency-dependent signal-to-noise transmission capability, which is described by DQE (detective quanta efficiency). In order to receive good values for these parameters, it is essential to bring the generated signal, namely the radiation generated by the scintillator, to the pixel matrix without any noteworthy attenuation (by loss processes and scattering processes) and without significant blurring of the location information (by scattering or refraction).
The best known method for the reception of signal information and location information is the direct vaporization of the pixel matrix, or the passivation layer with the scintillator layer fashioned on it. This is possible for solid state detectors that are composed of a single panel when specific temperature boundary conditions are maintained. Problems occur, however, if this technique is used for large-surface detectors that are comprised of a number of sub-detectors arranged side by side. Microscopically small fissures or cracks always arise at the butting edges of these sub-detectors, which immensely disturb the crystal growth of the scintillator layer and prevent a steady growth. Satisfactory signal transmission, and therefore satisfactory image quality cannot be achieved in the surrounding area of these butting edges. Therefore, heretofore a direct vaporization of such large-surface solid state image detectors has not been possible.
In order to still be able to deposit the scintillator on such a large-surface pixel matrix, the scintillator layer is initially vaporized on a substrate, normally a thin aluminum foil, and is subsequently glued to the sub-detectors of the solid state image detector with the scintillator layer directed toward the pixel matrix. The optical coupling properties are considerably impaired by this intervening adhesive so that MTF and DQE are worse compared to a direct process, particularly at higher spatial frequencies.
An object of the present invention is to provide a method that makes it possible to produce a large-surface solid state image detector with an improved optical coupling of the scintillator layer.
For solving this problem, in an inventive method of the type described, a layer which is transparent for the radiation emitted by the scintillator layer is deposited, on the passivation layer and the scintillator layer is subsequently vaporized on this deposited layer.
In the inventive method, a radiation-transparent layer is initially deposited on the passivation layer (silicon nitride, for example) of the pixel matrix that is composed of a number of adjacently arranged sub-detectors; an even (smooth) surface is also created in the region of the butt joints of the panel edges as a result. This makes is possible to directly vaporize the scintillator material on this layer as well as to have a homogenous scintillator layer grow in the region of the butt joints of the sub-detectors. Since the radiation-transparent layer is directly deposited on the passivation layer of the pixel matrix and since the scintillator layer is directly vaporized on this layer, extremely good optical coupling results.
In order to further improve the optical coupling, a layer, whose refraction index lies essentially midway between the respective refractive indices of the scintillator layer and the passivation layer or of the semiconductor material, can be inventively utilized. This keeps the refraction as low as possible in the coupling of the radiation generated by the scintillator layer into the radiation-transparent layer and the coupling of the radiation which penetrates this layer into the passivation layer or the semiconductor material.
In a first embodiment, a foil that is glued onto the passivation layer or that is adhesively fastened can be used as the transparent layer. An adhesive having a refractive index between the refractive indices of the foil and the passivation layer or the semiconductor material can be utilized. Since the adhesive layer also exhibits a specific thickness, a further improvement of the radiation coupling with regard to refraction can be achieved by suitable selection of the refraction index thereof. A foil that is formed by a number of foil layers that exhibit different refractive indices can be used.
The thickness of the foil should be as low as possible and should be within the range of a few xcexcm. Since the scintillator is vaporized directly on the foil it will be exposed to the temperatures present during the vapor deposition, thus it should exhibit in the case of Csl a temperature stability up to at least 180xc2x0 C., up to at least 250xc2x0 C., in particular.
In an alternative inventive embodiment to fashioning the layer as a foil, the layer is formed by casting a viscous layer of material with a subsequent hardening or curing of same. The sub-detectors, are cast from the top side, so that a very even surface layer (on which the scintillator material can be vaporized) is produced. An adhesive can be utilized as the layer material, with the thickness of the layer should also be a few xcexcm here. In the hardened condition, the utilized layer material should be stable up to at least 180xc2x0 C., particularly up to 250xc2x0C.
Especially in the case of utilizing a foil, it has proven expedient, for reasons of stability, to at least partially fill the butt joints of the adjacent sub-detectors, which are arranged on a glass substrate, with a filling material before the layer is deposited. In the case of casting the layer, the butt joints, in any case, are filled with the layer material, so that a sufficient stability is derived therefrom.
In order to avoid radiation being laterally coupled into the semiconductor material, and therefore into the pixels at the borders via the butt joints, the filling material should not be transparent for the radiation emitted by the scintillator layer. An adhesive is also appropriate for this purpose. In the case of utilizing a foil and in the case of fashioning a casting layer, this filling material, particularly the adhesive, is previously introduced into the butt joints, then the foil is deposited or the layer material is cast out. The butt joints should be filled as completely as possible with the material that is not radiation-transparent. Thus, an unintended coupling of radiation emitted by the scintillator can be avoided, which would lead to information falsification in the edge area.
Apart from the method, the invention also is directed to a solid state image detector, formed as a pixel matrix that is coated with a passivation layer and that is made of a semiconductor material, particularly an amorphous silicon and a scintillator layer for converting radiation incident on the scintillator layer, so that it can be processed by the pixel matrix, the matrix being composed of a number of sub-detectors that are arranged side by side and that are referred to as panels, and having a layer, which covers the passivation layer and on which the scintillator layer is deposited, that is deposited on the passivation layer.